Attempt to fix slices of vectorized locators

Create a 1D dataspace of the correct shape during datVec and then
allow datSlice to work as it normally would for any other type
of slice (use a hyperslab).
datUnmap: write to the correct pointer

The CNF registered pointer, not necessarily the same as the
start of the mapped region.
Experiment with converting HDF5 error code to HDS error code

Although H5Fopen does seem to have a limited number of states
and can't really tell between an error with the HDF5 file that
is being opened and the file itself not being in HDF5 format.
datVec: Disable vectorizing of a sliced locator
hdsOpen: H5Fis_hdf5 does not work if the file is open

So can't be relied upon (unfortunately the routine does not
look up to see if the file is already open by HDF5)